1. Field of the Invention (Technical Field)
The present invention relates to the fields of optical absorption spectroscopy and analytical chemistry.
2. Description of Related Art
Absorption of light by a chemical species inside a cell is a commonly employed method to measure the concentration of the species. The absorbance, A, which is defined as the logarithmic ratio of the incident light intensity, I0, to that transmitted through the sample, I, can be related to the absorbing species' concentration, c, using Beer's law:A=Log(I0/I)=σc I where σ is the cross section or extinction coefficient and I is the optical path length through the sample. Other materials in the optical beam path that decrease the transmitted beam intensity may limit the sensitivity to the species of interest.
One manner in which absorbance sensitivity for a particular species is commonly reduced is through the presence of optically scattering surfaces in the light path. When a light beam travels through a transparent material such as a window, back reflections and beam scattering occur. When light that has been scattered by one surface is again backscattered forward along the original beam path by a second surface, an optical cavity or etalon has been formed. If the light from this etalon reaches the detector, a reduction in the light intensity may be observed. This intensity reduction arises from destructive interference between the incident and reflected light beams. Interference occurs when the refractive index weighted length of the etalon is not exactly a half integer multiple of the light wavelength. In such a state, a non-resonant cavity exists. As the wavelength of the light is varied—perhaps to measure a spectrum—the etalon may periodically become resonant. Thus, the magnitude of the interference observed in a wavelength scan will oscillate, creating a periodically undulating background.
The overlapping reflections and scattering causing interferences can originate from the different surfaces of one transparent object or between the surfaces of different transparent objects. These interferences change the transmitted beam intensity and thus, change the overall absorbance. In high sensitivity absorption spectroscopy measurements, absorbances below 1 part in 105 can be measured. Frequently, the baseline oscillations created by interferences limit the minimum absorbance that can be attributed to the species being detected. Minimizing and/or eliminating these interferences is essential for increasing the measurement sensitivity to the species being detected.
When a bottle (container) is placed in the beam path, interferences from back reflections and beam scatter are caused by the bottle's walls. The magnitude of these interferences varies with the optical clarity of the bottle at the probe wavelength. The optical clarity of the bottle is determined by the bottle wall material as well as any material that is adhering to the bottle wall, either internally or externally. Due to variation in optical clarity across the bottle, some regions of the bottle wall will generate smaller interferences than others. In addition, the magnitude of the interference at a given wavelength is determined by the refractive index weighted distance between the reflecting/scattering surfaces. In the case where the inner and outer surfaces of a given bottle wall are causing the interference, the thickness of the wall is a factor in the magnitude of the interference at a specific wavelength.
Reducing the presence of interferences in optical systems has been the subject of many studies reported in the absorption spectroscopy literature. As noted below in the review of relevant patents, a variety of approaches have been tried. These approaches involve post measurement signal processing, varying the wavelength bandwidth of the light source, adding optical elements to the beam path, or mechanically moving parts of the optical system in order to reduce the signals caused by the interferences.
Aside from interference fringes, another problem encountered with performing absorbance measurements through containers is the presence of opaque materials adhering to the container walls either internally or externally in a non-uniform manner. For example, in a bottle containing liquid where headspace gas concentration is being monitored, spots or droplets of material may form on the container wall above the fill level. On the exterior of the bottle, miscellaneous material may exist. In addition, scratches may be present on the bottle. If the optical beam should be incident on these less transparent regions, greatly reduced or no light may reach the detector. The reduced signal level can be problematic in terms of reducing signal to noise levels or in terms of gain linearity. With low signal levels, electronic noise may become significant. For single wavelength measurements, the additional loss of intensity caused by spurious material will increase the observed absorbance. This added absorbance cannot be separated from that being generated by the species of interest inside the container. Thus, methods that eliminate or reduce absorbances from material other than the species being measured will increase the measurement sensitivity.
The following patents that address fringe reduction in optical absorption measurements do not teach the present invention or its advantages:
Silver and Stanton, “Laser Absorption Detection Enhancing Apparatus and Method”, U.S. Pat. No. 4,934,816, describe a method for improving optical absorption sensitivity by longitudinally vibrating an element of the optical system along the beam direction. The vibrating element must be contained within the etalon and cause the length of the optical cavity to oscillate by more than one quarter of the optical wavelength and preferable several wavelengths. The oscillation averages the magnitude of the interference over all phases of the incident beam. This averaging serves to reduce the sensitivity of the interference magnitude to slight changes in the etalon path length. It also eliminates the wavelength dependence of the interference. The result is that periodic oscillations caused by an etalon in a wavelength spectrum are greatly reduced. The present invention does not utilize a longitudinal vibrational motion of an optical element in the system, but rather utilizes variations in the bottle wall thickness and optical clarity to eliminate the interferences. The bottle does not translate with the present invention, but rather rotates.
James R. Veale, “Apparatus and method for nondestructive monitoring of gases in sealed containers”, U.S. Pat. No. 6,639,678, describes a system and method for measuring a gas inside a sealed container using absorption spectroscopy. A diverging beam is used in order to reduce scattering overlap with the incident beam and thus, reduce the interferences present in the measurement. The present invention does not put any requirement on the beam spatial characteristics.
Christopher R. Webster, “Method and apparatus for enhancing laser absorption sensitivity”, U.S. Pat. No. 4,684,258, uses an oscillating plate placed at Brewster's angle inside the interfering etalon to spoil the cavity. The plate is angularly dithered approximately 1 degree in order to oscillate the etalon path length. A problem with this method is that the plate will introduce new interferences in the system. The present invention does not utilize an optic placed at Brewster's angle or any additional optical components to spoil the etalon.
Klaus W. Berndt, “Methods for detecting microorganisms in blood culture vials”, U.S. Pat. No. 5,482,842, describes a method of measuring carbon dioxide in vials. A dual beam approach is taken in order to eliminate background effects from the vial walls. While this patent does not explicitly address interference fringes, it is concerned with background effects. This method does not utilize mechanical motion as a means of reducing background noise.
Whittaker et al., “Method and apparatus for reducing fringe interference in laser spectroscopy”, U.S. Pat. No. 5,267,019, describe a method for reducing fringes by modifying the wavelength bandwidth of the light source. This method utilizes a triangular wavelength modulation on the laser. The triangular waveform enhances the species signal relative to that caused by interferences. This method does not utilize mechanical motion as a means of reducing interferences.
The following patents that address performing measurements inside bottles are not concerned with reducing intensity interferences due to scattering or back reflections or with reducing absorbances caused by materials that are not the species being measured.
Sussman et al., “Detection of the presence of biological activity in a sealed container utilizing infrared analysis of carbon dioxide and apparatus therefor”, U.S. Pat. No. 5,155,019, describe a method and apparatus for measuring carbon dioxide in a bottle through absorption spectroscopy at 2300–2400 wave numbers. No method of reducing potential optical interference is employed by their invention.
Nix et al., “Method for testing carbonation loss from beverage bottles using IR spectroscopy”, U.S. Pat. No. 5,473,161, describe measuring carbon dioxide levels inside a bottle through infrared absorption spectroscopy at 4922 to 5034 wave numbers. Again, no method of reducing potential optical interference is employed by their invention.
Sternberg et al., “Photometer with Rotating Sample Container”, U.S. Pat. No. 4,372,683, disclose an improvement for light scattering type of measurements, including other scattering techniques such as fluorescence, luminescence, and scintillation. The disclosure does not relate to absorption spectroscopy, and so Sternberg et al. are not addressing etalon fringe issues or even seemingly transmission of the incident light through the bottle. Sternberg et al. are merely concerned with the background light scatter from the bottle that arrives at the detector and the attenuation of the scattered light from the sample.
Other arguably related inventions include Julius Z. Knapp, “Particle Detection Method for Detection of Contaminating Particles in Sealed Containers”, U.S. Pat. No. 5,694,221; John G. Brace, “Apparatus and Method for Noninvasive Assessment of Pressurized Container Properties”, U.S. Pat. No. 5,614,718; Manique et al., “Method and Apparatus for Inspecting Liquid-Filled Containers”, U.S. Pat. No. 5,523,560; Knapp et al., “Method and Apparatus for Inspecting Liquids in Transparent Containers”, U.S. Pat. No. 3,966,332; and Steven R. Hofstein, “System for Detecting Particulate Matter”, U.S. Pat. No. 3,830,969.